Polyurethane and polyurea polymers are well known in the art and have been in use for decades. With a wide variety of potential chemical structures and properties these polymers have found applications in many diverse fields. In recent years polyurethanes have been extensively used in biomedical applications due to their accepted biocompatibility.
The structure of a polyurethane polymer can be generally described as a copolymer with a soft segment and a hard segment. The hard segment generally has stiffer bonds than the soft segment, which is reflected in a higher glass transition temperature (Tg). The stiffness of the hard segment can be attributed to the presence of a urethane/urea group or oligomer. The molecular weight ratio of hard segment to soft segment has a critical impact on the mechanical properties of the final polymer as does the chemical nature of either segment.
Historically, in the preparation of polyurethane polymers, chemists have used polyethers as soft segments due to the mobility of their chains. Typical polyether polymers are polyethylene oxide (PEO), polypropylene oxide (PPO) or the higher homologues.
PEO and to a much lesser extent PPO can facilitate additional hydrogen bonding between segments thus yielding improved mechanical characteristics. Reduction in the ability of the soft segment to hydrogen bond with the hard segment has generally lead to poor tensile strengths.
Polyethers also impart significant hydrophilicity to polyurethanes but the effect decreases with higher polyether homologues. The propensity for these materials to undergo hydrolytic, oxidative and enzymatic degradation increases with hydrophilicity.
In more recent years alternative polymers have been used as soft segments to improve the biostability of polyurethanes but the mechanical properties of these have been limited.
The hard segment of a polyurethane polymer is the region rich in urethane or urea linkages formed from the reaction of diisocyanate molecules. These may be a single urethane or urea linkage or may be multiple repeat units covalently bound via urethane or urea bonds.
Diisocyanates can be divided into aromatic type and aliphatic type. The aromatic type is the preferred molecule. Aromatic diisocyanates yield bonds that have restricted rotation and thus low mobility due to their bulky nature. Such aromatic systems provide stiff chains to function as hard segments, which mechanically reinforce a polymer improving greatly the tensile properties of the polymer.
Aliphatic diisocyanates on the other hand typically form bonds of greater mobility thus resulting in hard segments which do not contribute greatly to the mechanical properties on the final polymer.
In recent years biostable polyurethanes have received a lot of attention and there are now several commercial products of this type that are designed for implantation in the body. These products tend to be solid elastomers of varying hardness.
In order to achieve improvements in biostability varying proportions of the conventional and chemically labile polyether soft segments have been substituted with more robust molecules. Examples of improved soft segment molecules include polyolefins, polycarbonates and polysiloxanes.
However, incorporation of such alternative soft segments into polyurethane formulations, reduces the ability of the soft segment to form hydrogen and other intermolecular bonds with neighbouring chains. This limits the elongation and tensile properties of the final material. In addition, because many of the new biostable soft segments are hydrophobic they induce incompatibility with aqueous reagents. Because foam formation in polyurethane chemistry involves reaction with water (water blowing) such reactions are not reliable currently with hydrophobic soft segments.
In summary, there is an emerging trend towards more biostable commercial polyurethane materials utilising unconventional soft segments. However, all of the emerging materials are elastomers and no foams are available. In addition, because many of the newer soft segment materials do not hydrogen bond well they lack mechanical characteristics which are desirable for some applications.